This invention relates to voltage regulators and more particularly to voltage regulators for aircraft generators.
Aircraft generator voltage regulators which regulate generator output voltage by controlling exciter field current are well known. These regulators generally utilize a feedback loop that includes a voltage sensor which produces a voltage signal proportional to generator output voltage, an amplifier which amplifies this signal, and a means for controlling exciter field current in response to the amplified voltage signal.
Aircraft generators (e.g., synchronous AC machines with brushless excitation) have requirements in common with generators for other applications but also have requirements for high reliability and performance in a compact and cost effective arrangement. The generator control unit, including the voltage regulator, is part of the generator system that must meet a high level of performance. While the subject matter of the present invention has particular advantage in aircraft generator systems, it is not limited to such applications.
Early regulators utilized series compensation techniques to improve transient performance. In these systems, a filter circuit was inserted in the feedback loop to compensate for the exciter time constant. However, circuit time constants vary with changing environmental conditions, such as temperature changes. This limited the effectiveness of series compensation circuits.
Minor loop feedback techniques were developed to achieve improved stability and transient response. In these systems, a signal proportional to the voltage regulator output voltage is fed back to the input of the amplifier in the major feedback loop. Although this results in improved performance, the circuit does not provide adequate performance for wide spread range systems, such as DC link systems with a converter in the closed loop.
Negative exciter field forcing is used to improve transient response time. This is accomplished by inserting an active current limiter, such as a transistor, in series with the exciter field coil. By turning off the transistor, a rapid reduction in field current is achieved.
If negative exciter field forcing is used in combination with a minor loop feedback circuit, the feedback signal which is derived from the voltage regulator output voltage cannot respond to the negative field forcing. The regulator then treats the negative field forcing as a disturbance, rather than a correction. Therefore transient response is degraded considerably.
The present invention incorporates an improved feedback technique which utilizes the exciter field current rather than voltage. The exciter field current accurately reflects the exciter output voltage and is therefore more useful in representing a state variable of the machine. Components of previous feedback circuits which attempted to model the exciter time constant have been eliminated since the time constant is accurately represented by the exciter field current.
A voltage regulator constructed in accordance with the present invention includes means for sensing exciter field current and for producing a signal proportional to the current. In addition, a voltage sensing means produces a second signal proportional to the output voltage of the generator. These two signals are combined to produce a third signal which goes to a comparator for comparison with a reference signal. The comparator produces a fourth signal in response to the difference between the reference signal and the third signal. A current controlling means responds to this fourth signal to control the exciter field current.